1. Field of the Invention
This invention relates to methods and compositions for treating subterranean formations. More particularly it relates to methods and compositions for treating a subterranean formation penetrated by a wellbore wherein a gel having a high viscosity formed from a metal-crosslinked polymer is injected into the wellbore or the formation through a suitable conduit. This invention specifically relates to a method and composition for reducing the viscosity of the gel.
2. Brief Description of Prior Art
Viscous treating fluids are commonly utilized in the drilling, completion, and treatment of subterranean formations penetrated by wellbores. Upon completion of the treatment, it is ordinarily desirable to remove the treating fluid from the well. To effectively remove the fluid, the viscosity of the fluid must be reduced. The reduction of fluid viscosity is referred to as "breaking" the gel.
A viscous treating fluid used in the treatment of subterranean formations is normally comprised of a polysaccharide or synthetic polymer in an aqueous solution which is crosslinked with an organometallic compound. Examples of well treatments in which metal-crosslinked polymers are used are hydraulic fracturing, gravel packing operations, water blocking, and other well completion operations.
In hydraulic fracturing applications a viscous treating fluid is used to transport propping materials to the formation. The treating fluid is introduced at a rate and pressure sufficient to produce a fracture or fractures in the subterranean formation and to extend the produced fracture or fractures from the wellbore into the formation. Upon the creation of the fracture or fractures, additional treatment fluid containing the propping materials can be introduced into the fracture or fractures in order to prevent complete closure. The propped fracture creates a conductive channel extending from the wellbore into the formation to facilitate the flow of oil and gas from the formation to the wellbore.
Upon completion of a treatment, it is desirable to reduce the viscosity of the treating fluid so that it can be recovered from the formation. Various materials known as "breakers" have been used to reduce the viscosity of the treating fluids. Acids, enzymes, and oxidizers are commonly used to reduce the viscosity of metal-crosslinked polymer fluids. These known breakers are somewhat effective to reduce viscosity by degrading polymer chains of the treating fluid; however, they are relatively ineffective against the metal-crosslinked portion of the polymer.
The present invention provides a method for improved degradation of metal-crosslinked polymer fluids. The method reduces fluid viscosity by (1) generating an acid which degrades the polymer chains and (2) producing a strong metal-ion chelator which removes the metal ion from the polymer to thereby uncrosslink the polymer.